BUG: avoid scalar statistics for structured Monte Carlo results

[ting-hong-shieh]

Pull request type

• Code changes (bugfix)
• Code maintenance
• ReadMe, Docs and GitHub updates
• Other

Closes #1145.

Current behavior

MonteCarlo.set_processed_results() sends all collected output values to NumPy's scalar aggregation functions. Equal-shaped lists and arrays are flattened into a single distribution, while ragged or empty values raise before results can finish loading.

At merge base cb6106a717207dd8fc2dfe1446d80ff75022f21b:

Input observations	Result
[[1, 2], [3, 4]]	scalar statistics with mean 2.5
two np.array([1, 2]) values	scalar statistics with mean 2.5
[[1], [2, 3]]	ValueError
[]	IndexError

New behavior

Summary statistics are calculated only when every observation is a real-valued scalar. Structured, non-numeric, boolean, mixed, and empty results remain unchanged in results; their existing five-element processed_results tuple is (None, None, None, None, None).

The tuple shape and scalar result behavior remain unchanged.

At head 0c93bf7058222e4997df52981d45a9da4ad25e17, all four inputs above return the five-None tuple without changing their raw values.

Verification

• .venv/bin/python -m pytest tests/unit/simulation/test_monte_carlo.py -k set_processed_results -q: 9 passed
• .venv/bin/python -m pytest tests/unit/simulation/test_monte_carlo.py tests/integration/simulation/test_monte_carlo.py -m 'not slow' -q: 50 passed, 5 deselected
• .venv/bin/ruff check rocketpy/simulation/monte_carlo.py tests/unit/simulation/test_monte_carlo.py: passed
• .venv/bin/ruff format --check rocketpy/simulation/monte_carlo.py tests/unit/simulation/test_monte_carlo.py: passed
• git diff --check: passed

The slow Monte Carlo simulation matrix was not run locally.

Environment: RocketPy 1.13.0; Python 3.12.6; NumPy 2.5.2; SciPy 1.18.0; pytest 9.1.1; Ruff 0.16.3; macOS 26.5.2 arm64.

Breaking change

• No

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.32%. Comparing base (e0ff281) to head (0c93bf7).
⚠️ Report is 53 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1146      +/-   ##
===========================================
+ Coverage    82.18%   83.32%   +1.14%     
===========================================
  Files          122      130       +8     
  Lines        16355    17076     +721     
===========================================
+ Hits         13441    14229     +788     
+ Misses        2914     2847      -67     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.